Anyone know how old they are?

Ikd but they're adults with 4 years age gap

Sayoung is 24 I guess and J is 28.

Pre time skip it was 20 and 16, currently its 28 and 24, Euijae the older one, sayoung the younger.

Mentally yes, but being stuck inside the dungeon and time being frozen, J stayed as 20 yo. While lsy got older at 24.

Well not really... you can see sort of how euijae is very traditional and described to be like a geezer. To me, throughout his character development and finding out his feelings with sayoung led him to be mentally 28.